You can research similar podcasts when starting your own podcast. You can go to www.itunes.com and download the software (click on podcast, then podcast directory in the bottom right hand corner, then search in the upper right hand corner). You can also check out sites like www.podcastalley.com and www.podcastpickle.com
I started a new podcast at www.datesfromhellshow.com and subscribed to other podcasts on online dating. I was surprised as time went on that most of my “competition” wasn't putting out new episodes. So if I had seen a large number of similar podcasts and decided “there is no room for me” I would've been wrong. While there may be numerous podcasts similar to yours, they may not be putting out any new content.
School of Podcasting: Expert Tips for Launching and Growing Your Podcast